Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, an optical assembly, a wafer stage assembly that retains a semiconductor wafer, a measurement system, and a control system.
Typically, the wafer stage assembly includes a wafer stage that retains the wafer, and a wafer mover assembly that moves the wafer stage and the wafer. Similarly, the reticle stage assembly includes a reticle stage that retains the reticle, and a reticle mover assembly that moves the reticle stage and the reticle.
The size of the images transferred onto the wafer from the reticle is extremely small. Accordingly, the precise relative positioning of the wafer and the reticle is critical to the, manufacturing of high density, semiconductor wafers.
In order to obtain precise relative positioning, the reticle and the wafer are constantly monitored by the measurement system. Stated another way, the measurement system monitors movement of the wafer stage and the reticle stage relative to the optical assembly or some other reference. With this information, the wafer mover assembly can be used to precisely position the wafer and the reticle mover assembly can be used to precisely position the reticle.
For example, for each stage, the measurement system can be an interferometer including an X system for monitoring the position of that stage along an X axis and a Y system for monitoring the position of that stage along a Y axis. Each system can include a mirror that is secured to the stage. If the stage is moved a long distance along the Y axis, the mirror for the X system on the stage needs to be relatively long. Unfortunately, the long mirror can be relatively heavy and can influence the performance of the stage. Additionally, the placement of the long mirror on the stage can interfere with the design of the stage.
In light of the above, there is a need for a measurement system that measures the displacement of a stage. Additionally, there is a need for a method and system for accurately measuring the position of the stage. Moreover, there is a need for an exposure apparatus capable of manufacturing precision devices such as high density, semiconductor wafers.